There are a number of possible encoding methods which may be used to compress audio files. Constant bitrate (CBR) encoding provides a constant rate output from a codec, i.e. a CBR encoder uses the same frame size for every frame. This may be beneficial when audio files are to be streamed across a medium of fixed bandwidth (e.g. over a wireless channel) because an audio file can be encoded at a bitrate which matches the available bandwidth. However as the nature of an audio stream is typically very non-uniform, such CBR coding techniques use more bits than are required for simple passages whilst being limited in bit allocation for complex passages. Where a particular frame has a complex sound in it, the encoder reduces the quality of the signal until it can be encoded in the available number of bits.
Variable bitrate (VBR) encoding however can respond to the complexity of any particular passage and allocate more bits to complex passages and fewer bits to less complex passages. Problems may occur, however, when streaming VBR encoded files because the resultant bitrate is unpredictable and the receiver may only have a limited buffer.
A compromise between CBR and VBR is average bitrate encoding (ABR). In ABR the encoder has flexibility in allocating bits to frames dependent on the complexity of the signal in any particular frame whilst maintaining a target average bitrate over a defined time period. This results in a higher quality signal than CBR and a more predictable bitrate than VBR. However, as the encoder does not know in advance which portions of the audio are more complex and therefore require more bits, some form of bit rate adjustment is usually required in order to ensure that the target average bitrate is achieved.